FIG. 8 is a block diagram illustrating a conventional digital satellite broadcasting system 101. A signal 102 transmitted from a satellite is received by a DISH antenna 103, and the frequency of the signal 102 thus received is converted by an LNB (Low Noise Block Converter) 104. The signal whose frequency has been thus converted is inputted to a satellite broadcasting receiving device 105. The satellite broadcasting receiving device 105 is, for example, a set top box (STB). An RF (Radio Frequency) signal 106 to be inputted to the satellite broadcasting receiving device 105 is firstly inputted to a tuner 107. Then, the signal level of the RF signal 106 is adjusted by an AMP 108 included in the tuner 107. The RF signal whose signal level has been thus adjusted is converted into a baseband signal by: a PLL (Phase-Locked Loop) 109 for outputting a signal having a local frequency F_lo; a 90-degree phase shifter 110 for shifting, by 90 degrees, the phase of a signal having a frequency F_lo; and a mixer 111 and a mixer 112 for mixing together (i) an RF signal whose signal level has been adjusted and (ii) a signal which has a frequency F_lo and whose phase has been shifted by 90 degrees. An LPF (Low Pass Filter) 113 and an LPF 114 remove, from the baseband signal, unwanted signal components outside a band. Then, the baseband signal is inputted to a QPSK (Quadrature Phase Shift Keying) demodulation section 116 included in a QPSK demodulation IC (Integrated Circuit) 115. The QPSK demodulation IC 115 is connected to an oscillator 127. The oscillator 127 provides the QPSK demodulation IC 115 with a standard frequency. Generally, the oscillator 127 is a crystal oscillator.
Generally, tuning to a desired signal is carried out as follows. A CPU (Central Processing Unit) 119, which is mounted in an MPEG (Moving Picture Expert Group) demodulation IC 118 included in a backend decoder 117, controls the QPSK demodulation IC 115. As a result, a PLL control signal 120 is transmitted, based on tuning information requested by a user, to a PLL 109 via the QPSK demodulation section 116. Then, tuning frequency information is set. The LPF 113 outputs a baseband signal (I signal) 121, and the LPF 114 outputs a baseband signal (Q signal) 122. Then, the baseband signal (I signal) 121 and the baseband signal (Q signal) 122 thus outputted are inputted to the QPSK demodulation section 116. After that, the baseband signal (I signal) 121 and the baseband signal (Q signal) 122 are subjected to digital demodulation, in response to a QPSK demodulation section control signal 123 transmitted from the backend decoder 117. Then, the signal thus obtained is outputted as a TS (Transport Stream) signal 124. The TS signal 124 is demodulated into a video/audio signal 125 by the MPEG demodulation IC 118.
In order to receive a signal, it is necessary to control the tuner 107 and the QPSK demodulation section 116 in accordance with the conditions of the signal to be received. A signal is received by (i) setting a tuning frequency and (ii) carrying out a signal search by the QPSK demodulation section 116. The tuner 107 and the QPSK demodulation section 116 are controlled by the CPU 119 mounted in the backend decoder 117. A control program to be used is the one which is stored in advance in: a memory 126 externally provided to the MPEG demodulation IC 118; or the like.
FIG. 9 is a schematic view of an RF signal to be inputted to the tuner. The horizontal axis represents a frequency. Within the frequency range (from 950 MHz to 2150 MHz) in which the tuner receives a signal, a signal TP (transponder) which has been subjected to QPSK modulation exists in accordance with a frequency table defined for each satellite. A symbol rate of each TP has a value within the range from 1 Msps to 45 Msps, according to the specification.
In regard to reception of a signal of satellite broadcasting, the following case may occur. Even from a single satellite, the number of TP signals which are receivable are different between the reception areas because every reception area has different conditions for a signal due to geographical factors and the like. In view of this, when an antenna and a tuner each of which is included in a digital satellite broadcasting system are initially set up, it is necessary to check, for each satellite, which channel can be received, that is, to carry out an automatic search. When a user watches and listens to a program, a channel is selected in accordance with the information obtained as a result of the automatic search.
The automatic search is carried out as illustrated in FIG. 10. That is, firstly a tuning frequency is set at the lower limit Fmin (950 MHz) of a reception frequency range. Then, the QPSK demodulation section 116 carries out a signal search around the tuning frequency which is thus set. The QPSK demodulation section 116 carries out demodulation by carrying out the signal search and reproducing a QPSK signal. The QPSK signal to be reproduced exists within a frequency acquisition range (±Fqpsk/2) for QPSK demodulation, which frequency acquisition range is set in advance. Also, the QPSK signal to be reproduced has a certain center frequency and a certain symbol rate.
The frequency acquisition range for the QPSK demodulation is generally set to a range of approximately ±5 MHz. This range is set in consideration of the following. The frequency of a signal (C-Band: 4 GHz to 8 GHz/Ku-band: 12 GHz to 18 GHz) transmitted from a satellite is down-converted by the LNB 104 so as to be within a reception range (950 MHz to 2150 MHz) of the tuner, the LNB 104 being mounted to the DISH antenna 103 for receiving the signal. At this time, a change in an outside temperature or the like may cause a drift in a local oscillation frequency F_lnb_lo, which is used by the LNB 104 during the down-converting. This causes an offset in the frequency to be inputted to the tuner after the conversion. Considering the assumed amount of the offset frequency, the frequency acquisition range for the QPSK demodulation is set to a range of approximately ±5 MHz.
Generally, in view of a functional capability of the QPSK demodulation section 116, it is possible to set a range wider than the above-mentioned frequency acquisition range. For example, a frequency acquisition range of the QPSK demodulation section 116, which is currently mounted in the QPSK demodulation IC 115 as a tuner, is ±Fmclk/2=100 MHz, that is, Fmclk=100 MHz, according to the specification. This means that the QPSK demodulation section 116 has the frequency acquisition range of ±50 MHz.
However, in an actual signal search, if the frequency acquisition range is set so as to be too wide, such a problem may occur that another TP signal adjacent to a TP signal which is to be acquired is acquired accidentally.
Furthermore, in a case where the frequency acquisition range is set so as to be wide, a problem may occur especially when a signal having a low symbol rate is received. The problem is as follows: (i) after conversion, a too large offset occurs in a frequency to be inputted to the tuner; and (ii) the signal having the low symbol rate cannot be locked properly. That is, it is possible to lock such a signal more surely with a frequency acquisition range which is limited in advance, compared to a case with a wide frequency acquisition range.
As described above, the frequency acquisition range for the QPSK demodulation is set so as to be narrower than a frequency acquisition range in which the QPSK demodulation section 116 actually can acquire a signal. When a signal exists within the frequency acquisition range for the QPSK demodulation, a reception signal is demodulated by the signal search carried out by the QPSK demodulation section.
The automatic search is carried out by judging whether or not a reception signal can be locked within the frequency range (from 950 MHz to 2150 MHz) of a signal that the tuner receives. That is, the automatic search is carried out such that: (i) a tuning frequency is set at Fmin; (ii) a signal search is carried out within the tuning frequency thus set; and (iii) the tuning frequency is shifted by a certain frequency step Fstep. For example, in FIG. 10, tuning to a frequency F101 within a frequency range RANGE 101 is carried out, and then a signal search is carried out with the range of the RANGE 101 set as the frequency acquisition range for the QPSK demodulation. At this time, a center frequency of a signal TP 101 exists within the frequency range RANGE 101. Therefore, the signal TP 101 is locked. Then, program information and the like in the signal TP 101 are read out and stored in a memory.
Next, in order to carry out the signal search within a frequency range RANGE 102, the tuning frequency is shifted by the frequency step Fstep, so that tuning to a frequency F102 is carried out. Then, in a similar manner as described above, the signal search is carried out, within the frequency range RANGE 102, around the tuned frequency F102 which is thus tuned. At this time, no signal exists within the frequency range RANGE 102. Therefore, tuning to a frequency F103 is subsequently carried out, and then the signal search is carried out within a frequency range RANGE 103. As such, the tuning and the signal search are repeatedly carried out until the signal search (i.e., sweeping) is carried out at the upper limit Fmax (2150 MHz) of a reception signal frequency range.
As well as the conventional digital satellite broadcasting system 101, there is a device for searching for a channel or broadcasting as disclosed in Japanese Unexamined Patent Application Publication, Tokukaihei, No. 6-85616 (published on Mar. 25, 1994). The device disclosed is a tuning device which carries out automatic preset by automatically searching for all channels and thereby increases an operation speed of the automatic preset carried out for all channels. Also, Japanese Unexamined Patent Application Publication, Tokukaihei, No. 10-145188 (published on May 29, 1998) discloses a receiving device which reduces the time taken for a channel search. Further, Japanese Unexamined Patent Application Publication, Tokukaihei, No. 10-313285 (published on Nov. 24, 1998) discloses a digital audio broadcasting receiving device which surely searches for receivable broadcasting and reduces the time taken for this process.
With the arrangement described above, the conventional digital satellite broadcasting system 101 carries out a signal search, within a reception frequency range, for every predetermined frequency step Fstep. In this case, however, every time tuning is carried out for each frequency step, it is necessary to ensure wait time for stabilizing a tuning PLL. The wait time is generally set at approximately 100 ms.
For example, such a case may be considered that a signal search is carried out, with Fstep=2 MHz, within a frequency range (from 950 MHz to 2150 MHz) of a signal that a tuner receives. In this case, the wait time is expressed as follows:(2150−950)/2*100=60000 [ms]32 60 secondsThat is, the order of one minute is spent as the wait time. Generally, the time taken for an automatic search is approximately five minutes. In a case where the wait time is the order of one minute, the wait time accounts for approximately 20% of the time taken for the automatic search. As such, the wait time is longer than the time taken for other operation in the automatic search. Thus, the conventional digital satellite broadcasting system 101 has such a problem that it takes much time for an automatic search.